This invention relates to a-point to multi-points communication systems and more particularly to an a-point to multi-points communication system suitable for improving efficiency of communication.
A prior art a-point to multi-points communication control scheme generally adopted in the field of radio communication is disclosed in, for example, JP-A-60-85631 and in this scheme, a-point to multi-points information or broadcast information is transmitted only unidirectionally from a transmitting station to receiving stations and no response is transmitted from the receiving stations to the transmitting station. This prior art scheme is applicable to a case where satisfactory quality of link can be insured and error rate of information can be so small that error of information is not problematic in operation. For example, this is the case where information of high redundancy such as video information in TV broadcasting is handled. Adopted in digital communication is a similar scheme in which error control information is added and an arrangement for information error detection and error correction is additionally provided for the sake of reducing error rate of information.
In the field of cable communication, in addition to a similar scheme to that for radio communication, a known a-point to multi-points communication control scheme as disclosed in JP-A-60-173993 has sometimes been adopted wherein in a-point to multi-points communication, responses from receiving stations to a transmitting station are transmitted on the time division basis. There is also available an a-point to multi-points communication control scheme as disclosed in JP-A-61-84140 wherein a transmitting station dedicated to a-point to multi-points communication is provided and this transmitting station fills the role of a different station (a-point to multi-points communication request station) in carrying out a-point to multi-points communication controlling.
As will be seen from the foregoing, in the prior art a-point to multi-points communication systems, an a-point to multi-points frame is transmitted from a station, that is, a master station and all of stations receiving the a-point to multi-points frame from the master station, that is, slave stations respond to the
(1) Transmitting no response frame.
(2) Transmitting a response frame each time the a-point to multi-points frame is received.
With the mode (1), reliability of information in each slave station can not always be insured. With the mode (2), on the other hand, in the event that abnormal receiving in a slave station leads to occurrence of retransmission, the slave station is not permitted to receive the next a-point to multi-points frame (taking a control scheme wherein frames can be transmitted successively within a stipulated number of frames without waiting for a response, for instance, a subsequent permissible successive transmission frame) until the master station completes the retransmission processing, thereby raising a fatal problem that the throughput of the a-point to multi-points communication system as a whole is degraded. As the number of slave stations increases, the frequency of occurrence of retransmission increases and the master station undertakes an increase in overhead of processing for reception of response frames, with the result that the above problem is aggravated to further degrade the throughput of the entire a-point to multi-points communication system.
As a countermeasure, a scheme has been proposed wherein in contrast to the scheme described in the aforementioned JP-A-61-84140, a representative receiving station is provided and slave stations other than the representative receiving station tap a-point to multi-points information which is being received by the representative receiving station, as disclosed in (A): "Proposal and Evaluation of Simplified High-reliability Broadcast Protocol for High-efficiency Transmission of a Large Amount of Data", Transactions of Information Processing Society of Japan, Vol. 27, No. 4, April, 1986, pp. 462-470.
Another scheme has also been proposed wherein each slave station does not normally make a response to a master station but only when receiving a request for response from the master station, a slave station in question or of interest transmits a response to the master station, as disclosed in (B): "Control Procedure of Data Link for Satellite Link", REVIEW OF THE RADIO RESEARCH LABORATORY, Vol. 32, No. 163, June, 1986, pp. 153-163.
The scheme described in the above (A) can insure reliability of information received by individual slave stations other than the representative receiving station if quality is uniform over individual links between the master and the individual slave stations but disadvantageously, can not insure reliability of information received by individual slave stations in a system in which the quality of link is irregular. In the scheme described in the above (B), timing for the master station to request each slave station to make a response is not clarified and a slave station of interest will not transmit a response indicative of normal receiving, thus preventing the master station from grasping the receiving state of each slave station. This impairs reliability of information in the a-point to multi-points communication system.